The present invention relates to a process for preparing alloys. More specifically, the present invention is directed to a process for preparing alloys of selenium and tellurium. One embodiment of the present invention is directed to an alloying process which comprises, in the order stated (1) heating in a reaction vessel a mixture of selenium and tellurium from ambient temperature to from about 270.degree. C. to about 330.degree. C. while maintaining the mixture in a quiescent state; (2) maintaining the mixture at from about 270.degree. C. to about 330.degree. C. until the entire mixture has reached substantial equilibrium with respect to temperature while maintaining the mixture in a quiescent state; (3) subsequently heating the mixture from the range of from about 270.degree. C. to about 330.degree. C. to the range of from about 500.degree. C. to about 580.degree. C. while maintaining the mixture in a quiescent state; (4) maintaining the mixture at from about 500.degree. C. to about 580.degree. C. until the entire mixture has reached substantial equilibrium with respect to temperature while maintaining the mixture in a quiescent state; (5) thereafter maintaining the mixture at from about 500.degree. C. to about 580.degree. C. for from about 0.75 hour to about 1.5 hours while vigorously agitating the mixture; (6) subsequent to agitation, reducing the temperature of the mixture from the range of from about 500.degree. C. to about 580.degree. C. to the range of from about 425.degree. C. to about 450.degree. C. while maintaining the mixture in a quiescent state; (7) subsequently maintaining the temperature of the mixture at from about 425.degree. C. to about 450.degree. C. for from about 4 hours to about 7 hours while maintaining the mixture in a quiescent state; (8) reducing the temperature of the mixture from the range of from about 425.degree. C. to about 450.degree. C. to the range of from about 290.degree. C. to about 350.degree. C. while maintaining the mixture in a quiescent state; and (9) removing the mixture from the reaction vessel.
The formation and development of images on the imaging surfaces of electrophotographic imaging members by electrostatic means is well known. One of the most widely used processes is xerography, described in, for example, U.S. Pat. No. 2,297,691 to Chester Carlson. Numerous different types of electrophotographic imaging members for xerography, i.e. photoreceptors, can be used in the electrophotographic imaging process. Such electrophotographic imaging members can include inorganic materials, organic materials, and mixtures thereof. Electrophotographic imaging members can comprise contiguous layers in which at least one of the layers performs a charge generation function and another layer forms a charge carrier transport function, or can comprise a single layer which performs both the generation and transport functions. These electrophotographic imaging members can also be coated with a protective overcoating to improve wear.
Electrophotographic imaging members based on amorphous selenium have been modified to improve panchromatic response, increase speed and to improve color copyability. These devices are typically based on alloys of selenium with tellurium and/or arsenic. The selenium electrophotographic imaging members can be fabricated as single layer devices comprising a selenium-tellurium, selenium-arsenic or selenium-tellurium-arsenic alloy layer which performs both charge generation and charge transport functions. The selenium electrophotographic imaging members can also comprise multiple layers such as, for example, a selenium alloy transport layer and a contiguous selenium alloy generator layer.
A common technique for manufacturing photoreceptor plates involves vacuum deposition of a selenium alloy to form an electrophotographic imaging layer on a substrate. Tellurium is incorporated as an additive for the purpose of enhancing the spectral sensitivity of the photoconductor. Arsenic is incorporated as an additive for the purpose of improving wear characteristics, passivating against crystallization, and improving electricals. Typically, the tellurium addition is incorporated as a thin selenium-tellurium alloy layer deposited over a selenium alloy base layer in order to achieve the benefits of the photogeneration characteristics of SeTe with the beneficial transport characteristics of SeAs alloys.
One method of preparing selenium alloys for evaporation is to grind selenium alloy shot (beads) and compress the ground material into pellet agglomerates, typically 150 to 300 milligrams in weight and having an average diameter of about 6 millimeters (6,000 microns). The pellets are evaporated from crucibles in a vacuum coater using a time/temperature crucible designed to minimize the fractionation of the alloy during evaporation.
U.S. Pat. No. 3,723,105 (Kitajima et al.), the disclosure of which is totally incorporated herein by reference, discloses a process for preparing selenium-tellurium alloys which comprises heating a mixture of selenium and tellurium containing 1 to 25 percent by weight of tellurium to a temperative not lower than 350.degree. C. to melt the mixture, cooling the molten selenium and tellurium gradually to around the melting point of the selenium-tellurium alloy at a rate not higher than 100.degree. C. per hour, and then quenching to room temperature within 10 minutes.
U.S. Pat. No. 4,822,712 (Foley et al.), the disclosure of which is totally incorporated herein by reference, discloses a process for controlling fractionation by crystallizing particles of an alloy of selenium which comprises providing particles of an alloy comprising amorphous selenium and an alloying component selected from the group consisting of tellurium, arsenic, and mixtures thereof, said particles having an average size of at least about 300 micrometers and an average weight of less than about 1,000 milligrams, forming crystal nucleation sites on at least the surface of the particles while maintaining the substantial integrity of the particles, heating the particles to at least a first temperature between about 50.degree. C. and about 80.degree. C. for at least about 30 minutes to form a thin, substantially continuous layer of crystalline material at the surface of the particles while maintaining the core of selenium alloy in the particles in an amorphous state, and rapidly heating the particles to at least a second temperature below the softening temperature of the particles, the second temperature being at least 20.degree. C. higher than the first temperature and between about 85.degree. C. and about 130.degree. C. to crystallize at least about 5 percent by weight of the amorphous core of selenium alloy in the particles.
U.S. Pat. No. 4,583,608 (Field et al.), the disclosure of which is totally incorporated herein by reference, discloses heat treatment of single crystal superalloy particles. In one embodiment, single crystal particles are heat treated by using a heat treatment cycle during the initial stages of which incipient melting occurs within the particles being treated. During a subsequent step in heat treatment process substantial diffusion occurs in the particle. In a related embodiment, single crystal articles which have previously undergone incipient melting during a heat treatment process are prepared by a heat treatment process. In still another embodiment, a single crystal composition of various elements including chromium and nickel is treated to heating steps at various temperatures. In column 3, lines 40 to 47, a stepped treatment cycle is employed in which an alloy is heated to a temperature below about 25.degree. F. of an incipient melting temperature and held below the incipient melting temperature for a period of time sufficient to achieve a substantial amount of alloy homogenization.
U.S. Pat. No. 4,484,945 (Badesha et al.), the disclosure of which is totally incorporated herein by reference, discloses a process for preparing chalcogenide alloys by providing a solution mixture of oxides of the desired chalcogens and subsequently subjecting this mixture to a simultaneous coreduction reaction. Generally, the reduction reaction is accomplished at relatively low temperature, not exceeding about 120.degree. C.
U.S. Pat. No. 4,414,179 (Robinette), the disclosure of which is totally incorporated herein by reference, discloses a process for preparing a selenium alloy comprising heating a mixture comprising selenium, arsenic and chlorine to a temperature between about 290.degree. C. and about 330.degree. C. to form a molten mixture, agitating the molten mixture to combine the components, continuing all agitation, raising the temperature of the mixture to at least 420.degree. C. for at least about 30 minutes and cooling the mixture until it becomes a solid. This alloy may be vacuum deposited.
U.S. Pat. No. 3,785,806 (Henrickson), the disclosure of which is totally incorporated herein by reference, discloses a process for producing arsenic doped selenium by mixing finely divided selenium with finely divided arsenic in an atomic ratio of 1:4 and thereafter heating the mixture in an inert atmosphere to obtain a master alloy. The master alloy is then mixed with molten pure selenium to attain an arsenic content of between 0.1 and 2% by weight based on the selenium vaporizable alloying component on the substrate. Examples of vaporizable alloying components include selenium-sulfur and the like, and examples of vaporizable alloying components having relatively low vapor pressures which include tellurium, arsenic, antimony, bismuth, and the like. Examples of suitable evaporation retarding film materials include long chain hydrocarbon oils, inert oils, greases or waxes at room temperature which flow readily at less than the temperature of detectable deposition of the vaporizable alloying components having higher vapor pressures in the alloying mixture. Examples of retarding materials include lanolin, silicone oils such as dimethylpolysiloxane, branched or linear polyolefins such as polypropylene wax and polyalpha olefin oils, and the like. According to the teachings of this patent, optimum results are achieved with high molecular weight long chain hydrocarbon oils and greases generally refined by molecular distillation to have a low vapor pressure at the alloy deposition temperature.
Of background interest with respect to the preparation of selenium alloy photoresponsive imaging members are U.S. Pat. No. 4,780,386, U.S. Pat. No. 4,842,973, U.S. Pat. No. 4,894,307, U.S. Pat. No. 4,554,230, U.S. Pat. No. 4,205,098, U.S. Pat. No. 4,609,605, U.S. Pat. No. 4,297,424, U.S. Pat. No. 4,609,605, U.S. Pat. No. 4,297,424, U.S. Pat. No. 4,554,230, U.S. Pat. No. 4,205,098, U.S. Pat. No. 3,524,754, U.S. Pat. No. 4,205,098, U.S. Pat. No. 4,710,442, U.S. Pat. No. 4,585,621, U.S. Pat. No. 3,524,754, U.S. Pat. No. 4,015,029, U.S. Pat. No. 3,911,091, U.S. Pat. No. 4,710,442, and U.S. Pat. No. 4,513,031, the disclosures of each of which are totally incorporated herein by reference.
Although known materials and processes are suitable for their intended purposes, difficulties remain with selenium alloy photosensitive imaging members with respect to negative shock electrical residual voltage. Negative shock electrical residual voltage refers to the cumulative imaging member electrical residual voltage generated after alternating sequences of positive and negative charge cycling. When selenium alloy imaging members are repeatedly charged in successive imaging cycles, the residual voltage remaining on the member subsequent to discharge and prior to the next charging step tends to increase with each cycle, particularly when the cycles follow each other in rapid succession and the residual charge has no time to leak away prior to the next imaging step. It has been observed that when an alloy imaging member is charged first to one polarity, then to the opposite polarity, and subsequently to the first polarity, either with positive and negative charging being performed alternatively or with positive charging being performed for a set number of cycles, followed by negative charging for a number of cycles and then followed by positive charging again, the accumulation of residual voltage on the imaging member is significantly higher than that observed when the member is charged only to one polarity. This increase in residual voltage observed when the imaging member is charged to both polarities is referred to as negative shock electrical residual voltage. Excessive residual voltage build up in the imaging member with repeated cycling reduces the dark development potential of the member, since the difference in potential between the charged and discharged areas is reduced as a result of the increased residual voltage. Negative shock electrical residual voltage can induce image defects such as poor image contrast, undesirable development of background areas, black banding (the undesirable development of linear gray or black strips in background areas), and the like. Accordingly, a need remains for processes for preparing alloys of selenium that, when incorporated into an imaging member, reduce negative shock electrical residual voltage. A need also exists for processes for preparing alloys of selenium that, when incorporated into an imaging member, reduce image defects such as poor image contrast, background deposits, and black banding. Further, there is a need for processes for preparing imaging members that exhibit reduced negative shock electrical residual voltage and reduced image defects resulting therefrom. There is also a need for processes for preparing alloys of selenium that, when incorporated into an imaging member, reduce negative shock electrical residual voltage and reduce image defects while retaining good photosensitivity and dark development potential characteristics.